1. Field of the Invention
This invention relates to the field of medical electronics and particularly to a bipolar electrode adapted for being electrically and structurally connected to body tissue such as the heart.
2. State of the Prior Art
Electrical stimulation of body tissue and organs as a method of treating various pathological conditions is becoming quite common-place. Such stimulation generally entails making some type of electrical contact with the body tissue or organ. In particular, with respect to the heart, electrical leads have been thoracotomy in which an electrode formed on the end of the lead are physically implanted into the myocardial tissues.
Various electrode structures and various techniques for implanting those electrode structures into such body tissue as the heart or myocardium, have been developed. Typically, electrodes attached to the heart are stimulated by a cardiac pacemaker which may be implanted within the patient's body. Previously, a thoractomy was commonly required to attach the cardiac pacemaker leads to the heart, and the electrical leads were sutured into electrical contact with the heart. This technique has numerous disadvantages. Firstly, a thoracotomy, which requires a large incision in the chest or thorax, is drastic surgery and has a relatively high mortality rate. Secondly, suturing the electrical leads into electrical contact with the heart causes severe trauma to the heart, which it is desirable to minimize.
An intravenous connection has also been used to attach electrical leads of a cardiac pacemaker to the heart. In this technique, the electrical lead is passed through a vein into the heart where it is held by fibrilla located in close proximity to the heart valve through which the lead is passed. There are, however, many disadvantages to this technique also, including: the possibility of damage to the vein during insertion, such as vein perforation; the failure to attach securely the electrical lead to the heart; the possibility of perforating the heart wall with the electrical lead during insertion or after attachment has been completed; and the possibility of improper lead placement in the heart.
Other techniques have included the percutaneous insertion through the chest wall or an open wound by means of a hollow needle with the subsequent placement of the electrode into the myocardial tissue. Still another technique involved the deformation or flattening of one convolution of a rigid helix serving as the electrode so that a keyed stylet could engage the deformed convolution to permit the electrode to be screwed into the myocardial tissue. However, this technique requires that the stylet be in physical contact with the helix during insertion into the myocardium and in addition has the undesirable effect of imparting torque to the proximal end of the coiled conductor.
In U.K. Pat. No. 1,277,107, there is described an electrode taking the form of a helically-shaped member and a tool for rotating the helically-shaped electrode whereby it is screwed into body tissue, e.g. the myocardium. In one disclosed embodiment, two such helically-shaped electrodes are inserted into the heart, whereby stimulating pulses are applied thereto from a cardiac pacemaker implanted within the patient's body. Further, there is disclosed an electrode assembly whereby two helically-shaped electrodes are intertwined between each other. Each of the helically-shaped electrodes has tipped portions whereat the electrical insulating material is removed, with the electrically-bared portions of the electrodes displaced from each other whereby a field is established therebetween.
In U.S. Pat. No. 3,737,579, assigned to the assignee of this invention, there is disclosed a unipolar body tissue electrode comprising an uninsulated, conductive, rigid helix adapted for attachment to body tissue and a flexible insulated conductor having a proximal end adapted for connection to a power supply and a distal end for connection to the helical electrode. In electrodes particularly adapted for use with cardiac pacemakers, there are particular problems heretofore unresolved.
Further, it is desired to use a plurality of electrodes to tend to prevent arrhythmias such as described in the above-identified Funke patent application. In particular, it would be undesirable to use two distinct electrodes at each point of stimulation or detection in that the number of electrodes as well as the surgical steps required to implant a plurality of electrodes are increased. As the number of electrodes is increased, the size of the surgical opening into the patient's body, the number of wounds into the heart and the resultant trauma of the entire surgical procedure are increased.
It is contemplated that the bipolar electrode structure including the double-helix electrodes of U.K. Pat. No. 1,277,107, could be utilized whereby the number of electrodes required for multi-electrode stimulation would be reduced. However, there are several significant problems resulting from the use of such a structure. First, the electrical insulation separating the intertwined electrodes may tend to deteriorate with age, especially within the environment of a living organism. The second of the intertwined electrodes having an unshielded portion displaced from the tip of the forwardmost helical electrode has a limited exposed area that is disposed at the epicardium of the heart. It is expected that after such an electrode structure has been implanted within a patient for several years that the surrounding portion of the myocardium would become ischemic or infarcted, thus blocking or reducing the electrical field established between the exposed portions of such an electrode structure. Eventually, it is contemplated that the resultant electric field would become so weak to be incapable of stimulating the heart, especially as the energy level of the power source, e.g. a battery, attenuates with extended life.
A further problem associated with cardiac pacemakers relates to the dissipation of the pacemaker batteries with extended life. After a period of time, e.g. 2 to 4 years, the implanted pacemaker must be removed and its batteries replaced. Therefore, it is desirable to reduce the current drain by appropriate design of the pacemaker's electrodes, whereby the battery life is extended, while maintaining the strength of the resultant electric field through the myocardium at a sufficiently high level to stimulate the heart.